Methods and apparatus for measuring antenna radiation patterns



Jan. 19, 1965 H. E. sHANKs ETAL.

METHODS AND APPARATUS FOR MEASURING ANTENNA RADIATION PATTERNS FiledJuly 17. 1961 4 sheetsl-Sheet 1 MM Mu IIJ www

NSWAM NNN Jan. 19, 1965 H. E. sHANKs ETAL 3,166,748

METHODS AND APeARATus FDR MDASURTNG ANTENNA RADIATION PATTERNS FiledJuly 17. 1961 4 Sheets-Sheet 2 4' 5 6 l7 www 0F f fP/W //v mf IP44/57am7a Pfff/mvg WWK/5 muse/smv@ .sw/raw 431 l T fvrewN-/ INVENTOR,

H. E. sHANKs ETAL. 3,166,748

4 Sheets-Sheet 5 INVENTOR.

PQJ EJE ffO/fP/V/ Jan. 19, 1965 METHODS AND APPARATUS PoR NEAsURTNGANTENNA RADIATION PATTERNS Filed July 17. 1961 AZR'.

Jan. 19, 1965 H. E. sHANKs ETAL METHDDS AND APPARATUS F'OR MEASURINGANTENNA RADIATION PATTERNS 4 Sheets-Sheet 4 Filed July 17. 1961 n wwwfar zone, or within the Fresnel zone.

' hofer diffraction pattern at distances denoted as 3166748 o METHODSAND APPRTUS non mAsUnusG Y ANTENNA 'RAmArroN PATTERNS .Howard E. Shanks,San Marino, and Robert W. Biclrmore,

lThis invention relates to methods and apparatus for measuring theFraunhofer diffraction pattern or far zone characteristics of an antennaoutside the far zone. Antennas have become larger and larger wherebytheir far eld distances may vary typically from several miles to tens ofmiles, thereby rendering the accurate determination of the far eldcharacteristics of these large antennas, in particular, very diflicult.Such large antennas are presently being used for satellite trackingpurposes and the like. In the past, relatively small antennas have beenemployed and the radiation patterns have been simply determined bymeasuring the pattern at the desired receiving point or points, eitherin the near zone, However, with the advent of the larger antennas and,therefore, the larger antenna ranges, the distance between atransmitting and receiving antenna may include various obstructions and/or cavities which cause the conventional'techniques to .be eitherunusable or very difficult to practice. The need for accuratelydetermining the far field radiation or the Fraunhofer diffractionpattern, in particular of large antennas, is'even a greaternecessitythan heretofore.

v In general, for most measurements, it has been found sucientlyaccurate to measure the far field or Fraunwhereinl) is the diameter ofthe antenna and A is thev wave length of the radiated energy.

The attempts to discover means of measuring the I Fraunhofer diffractionpatterns of very large antennas I* molding ofthe aperture surfaces intothe segment of a sphere.Other known'methods require measuring theamplitude and phase of the radiated field over a closed surfacesurroundingk all sources. is much larger than the antenna, i.e., Fresnelregion, a computer.` can be designed which performs a Fresnel-Fraunhofer transformation to obtain the/Fraunhofer The difculties ofprobing fthe aperture eld and measuringV phase .over tremendous pathlengths, howpattern.

I ever, are well' known and make this method impractical.

Accordingly, a measuring method for determining the AAfartfield orFraunhofer diffraction pattern is desired that eliminatesythe need for adirec-t phase measurement and Y which does not require modification ofthe antennal from V"its f normali operation 4configuration. Themeasuring method.-should,'inaddition, be equally applicable to fixedtances involved should notlbe overly critical.

The present invention provides improved'methods and apparatus formeasuring the Fraunhofer diffraction p at` f tern outside the far zonewithout requiring modification If the closed surface v United StatesPatent 3,166,748 Patented Jan. 19, 1965 of the antenna and without thenecessityA of either probing the field at ve'rygreat distances ormeasuring both amplitude and phase at smll dis-tances or a computertypeoperation on a near zone measuredvalue to `obtain the far zonevvalue.The present invention, therefore, advantageously provides 'a practicaland expedient method and'apparatus .of measuring directly the Fraunhoferdiffraction pattern of any antenna, particularly Very large antennas. t

In general, the method of the invention includes the measuring of theFraunhofer pattern of an antenna, preferably in the Fresnel zone, byradiating electromagnetic energy from a source and receiving theradiated energy at a preselected range or distance outsideof theFraunhofer zone in a periodic time-modulated fashion and -thenelectronically processing the thus modulated signal to directly derivethe Fraunhofer pattern of the antenna. The processing of the timemodulated signal may include the voltage multiplication or simulatedmultiplication of this signal to cause the received radiated signal toassume a plurality of unique information portions, including onecharacteristic of the Fraunhofer pattern and then separatingvout onlythat signal characteristic of the desired pattern.

The apparatus for providing this direct measurement of the Fraunhoferpattern in a near zone may include a plurality of receiving antennas orprobes spaced apart in a predetermined fashion and controlled forpreselected intervals to cause the radiation coupled to a receiver toassume a time-modulated characteristic. In Vone -particportion ,i ularembodiment `of the invention, the receiving antennas are spaced inaccordance-withy thev trigonometric cosineVV function and the antennasare coupled to the receiver for substantially equal time increments andare switched on and off in a sequential fashion-to cause the radiatedenergy delivered `to the receiver to have -the desired time modulatedcharacteristic. In order to separate the-plurality of unique informationportionsfwhich comprise this time modulated signal, a voltagemultiplication techniqueis required. In ythe preferred embodiment, eachreceiving antenna includes an attenuator and a phase shifter in seriestherewith to cause the signals from these antennas to assume acontrolled amplitude characteristic similar to that performed by aconventional voltage multiplication'l arrangement.; When the signalhaving lthis separable characteristic is applied to a receiver thesignal may then be further processed by means of mixing it with a'flocaloscillator `signal and applying it to a filter' tuned merely to thevoltage portion characteristic of the Fraunhofer pattern. .Uponseparating out thev desired voltage portion, it may be applied to adetector and a conventional display device lsuch as a .pattern recorder`i to directly derive and display the amplitude characteristic of the farzone pattern.

These and other features of the present invention may i be more fullyappreciated when considered in the light of the following specificationvand drawings, in which;

FIGURE. lis a block diagram of the general system embodying theinvention; t

FIGURE 2 is -a graphical illustration-of the relativev amplitudes of thesuccessive terms of the-Axial Fresnel 1 Field Expansion;

FIGURE-3 isla graphical illustration of an arrange-yv ment of areceiving array showing the modulation of the f range with time;

FIGURE '4 is afgraphy illustration of the relative accuracies of priorart unmodulated measurement methods and -themodulated.measurementfinethod ofthe present invention under `different-'conditions; f

' Y,FIGURE 5 iseagluock'diagram 'or-an embqdifiint of 'i G3 FIGURE 6is adetailed schematic diagram of a particular arrangement of the invention`of FIG.

In order to process the time modulated signal to` derive the far zoneinformation, however, the signal must be FIGURE 7 is a schematic diagramof Calibrating arl rangement forV the embodiment of FIG. 5;

FIGURE 8 is a block diagram of an embodiment Vof the invention of FIG.vl based on ,a Doppler arrangement employing a non-vibrating receivingprobe; andV FIGURE 9 is a block diagram of an embodiment of theinvention of FIG. l based on a Doppler arrangement 11:0 in the regionfor which lR] c, where c is the radius of a sphere which just bounds thesource and R, 0, Yare the usual spherical coordinates, and R is therange or distance between the transmitting and receiving antennae. Inparticular,v at a point P(R,0, )"external .tothe bounding sphere, thevoltage may be expressed as:

' e-ikR Bn@ qs) .m

EP(R,0,) R,T6 Y (2) n=0 where it mayl beshown that the n'=0 te'rm is theFraunfurthermo'diied to allow the far zone term to'be readilyseparable.V This further processing may be accomplished Y by controllingthe amplitude of eachterm in the above Expression 6 and which amplitudecontrol may be aecomplished by a voltage multiplication technique. The

time modulated Asignal isV multiplied by a voltage that is synchronousor time related to the voltage represented vby EquationS but notnecessarily of the same amplitude. VThis voltage yexpressed byExpression 7 as follows:

andthen substituting the Fourier series form for F(t). The resultingvoltage is The highestharmonic in this equation (neglecting thedependence) cosine (MNM), has associated with it the vcoeliicientB0(0,), the Fraunhofer yfield, the desired signalportion to beseparated.

With rthe above analysis in mind, it will be noted that one of theVelementary parameters which must be estab-A lished for the Vmethod ofthis invention for any given hofer field or far zone and the remainingterms comprise the Fresnel zone or near eld If the above equationslareapplied to 'a bounded source, p suchas an antenna, mounted onV`arotating pedestal and f a pickup probe is located in the near zone ofthe antenna andthe distanceV of the probe from the center of rotation Yis periodically variable with time.

Under these conditions, theidistance, 'or 'range R, maybe expressed as jR=Rot1+tF 01 y '(3) where RK, is the mean value of R, a constant, )t isthev modulation coeicient, and'F-(t) is.the'time-periodicfunction. Underthese `conditions the voltage at the probe terminal is y [Huren-@+12'where the series maybe terminatediat a value of 'n=N if analloWableerr-or is prescribed. If the phase term Fis eliminated and thefrequency converted from R-F to I-F, to ease the processing of thereceived signal, Equation 4 has the form i Equation may be expressed asfollows:

. l approximate vrange zis yone mile. iseries in powers of physicalsituation is the value of N, the number of terms which must `be used in`the above expressions to Vobtain reasonable, accuracy. N is,iin general,a function primarily of the type of aperture distribution and the radialcof ordinate R. The determination of lthe number of terms ymay best befound by Vconsidering a particular physical arrangement, such las theantenna presently/located at the GoldstoneTi-acking Facility of thel-IetPropulsion Labo-,- A ratorypPasa'dena, California. boloid of revolutionwith a`diameter of u85.3feet.

This antenna is a para- The antenna has anV approximate taper of(14,002) (-24.6 db sidelobe level). The frequency radiated from this,ntenna Will'be consideredto be 2400'megacylcles and the v It 'can beshown that la' y correspondingto the multiple Sommerfeld expansion of YThis expression canbe evaluated for the yfrequency of Vegt/0 Z 5151*@[1+ @www nos una (5) Since F(t).. `is periodic, (may be represented byVa, 'Fourier series, ,Y 1 i Y E. I

This indicates that each term including the terni repi resentativeof theFratmliofer` jiield of the-above Sommer-Y Y for- Rv in lanes.

V2.400 megacyvcles 'and a'=42.5 feet' for the physical situation we haveassumed and it becomes las follows:

of the successive terms ingthe above seriesfor a range of,

onemile. lt `will bejseen that the series convergesfrquite feldexpansionhas a unique vari-ation'` with-range change v vor detection the signalpo v A. ...includingthe Fraunhofer field may fbe separated and readingof the' Fraunhofer` pattern. l:

Thesev terms, then, cani,

ldisplacedto, give af direct'` and thereforemay'flbe,consideredfaperiod-ic .timemodula- .l tion oftheVreceived`radiation. be considered as a. pluralityof'uniqueinformation-chan-V nels orfsignal portions` andwith,proper's,ignal processing power). The; above expressin y g to.determine 'the vvalue gNinfEqua-,tion 5 for a `prescribed'allowable*error.:L,` Y Y l J l rapidl'ywat this frange; the amplitudeVof n=8s term is'ap`Y` I! lproxlrnz'tely 0.1 percent 0f that-for thelwz() Vterm,:lorthe x Y l yiield,fO-.Olfpercent in Fraunhofer temi 0:1percent can, therefore', ,be use d {The method of measuring.theiFraunhofen pattern outv sideof the, Fraunhofer zone is based uponthe general.kr l

YFIGURE 2. shwrjggtaphiaiiy the' relativa mentada-SQ tioncharacteristic.

Y -distributed over the range interval. switching time increments foreach receiving antenna or theory of a periodic time variation of theVrange of the total eld being measured. This general theory is discussed,in an article by H. E. Shanks and R. W. Bickmore appearing in theICanadian Journal of Physics, volume 37, 1959, on pages 263-275 and isentitled Four- Dimensional Electromagnetic Radiators. As is pointed outin this article, the time domain is utilized as an additional variablewith which to control the antenna radia- The radiation characteristic isoontrolled by periodically time modulating one or more of the antennaparameters to cause the radiation pattern to have a characteristic whichis periodically changing as a function of time to provide a number ofindependent information channels corresponding to the harmonic frequencycomponents of the modulated patterns. This, then, is effectively a timemodulation of the measurement distance and which `range has beenexpressed hereinabove in Equation 3. The time modulated characteristiccan be accomplished by physical movement of the probe antenna over themeasurement range. This is not a very practical method Vfor allapplications and a very good approximation to the. continuous motion ofthe probe may be Obtained by sequentially switching along a series ofprobe antennas Assuming equal probe, it can be shown that the choice ofa cosine trigonometricl distribution of the probe antennas over therange interval results in thebest suppression of the harmonies appearingin the FU). This condition is illustrated in FIG. 3., wherein theprobespacings are unequal and approximate the fundamental cosinusoidalvrange variation and which cosinusoidal range variation may be obtainedwitha relatively few elements. In one kexample, only .17 elements wererequired.

the'fphysical array of 'receiving antennas or probe elements'twice, oncein each direction, to obtain the periodic variation or cosinusoidalapproximation. In addition, the physical array may contain either an oddor an even number of elements. In the above approach, an even expressionis substituted in Equation 8 `shown above,V an.l axial. signal level maybe obtained and the expression 'of the summation of the expressions ofvfor n 'Nv and, therefore, represents the error contribution for a givenvalue of N. This expression, therefore, forms the-criteria for thefinal. determination of the value of N foi aprescribed allowable errorin the measurement of the'Fraunhofer field. It should be noted that adistinct advantage inaccuracy overthe expression in Equation fisobtained Isince Bg, for n between l and'N-i-l do not appearl" Themeasurement accuracy which could be obtained ,for N equal to 2,13, :and4 for a range of one mile v compared, with theaccur'acy of prior artmethods in FIG at measurementdistances of Y :beiseefn anlexamination-ofEG. 4 'that with It should be noted l that a periodr of the rangevariation requires traversing N =3 an accuracy at broadside is achievedwhich is better than the eld measured at or at the conventionalmeasuring distance. lt has also been noted that the value of the eldwhich is measured in the customary manner at a range of is in error byapproximately 17 percent. It can now be concluded, therefore, thatEquation l2 also forms .the basis for the determination of the level ofthe voltage which is proportional to the `far field term BO(O).

Another factor that m-ust be determined is the required receiversensitivity as the function of the modulation coefficient. lt has beenfound that it is desirable to choose N `as small as is consistent withreasonable accuracy and that the required receiver sensitivity mustincrease 4as the number of terms Ibecomes greater and as the rangedecreases. Therefore, although the concept of this invention isapplicable to measurements in both the near Zone and the Fresnel Zone,it is best applied in the Fresnel region.

Now with the above analysis in mind, the general system for measuringthe Fraunhofer pattern will be discussed, that is, a relatively largeantenna will be assumed as the transmitting antenna and the measurementswill be taken at a distance which is a fraction of the usual far fielddistance for vthe antenna. `The system employs a method of processingthe signal from a receiving antenna whose distance from the.transmitting antenna-is varied periodically with time in such a fashionthat the far eld term in the aboveSommerfeld multiple expansion of thefield can be separated from the near field terms. This processing .orseparation involves the multiplication or" age which is related tothetime variation of the range and filtering out the resulting signal toobtain the harmonic which has the far field dependence of thetransmitting antenna as a coeflicient.

The general system is shown in FIG. l wherein the transmitting antenna10 is shown coupled to a radio frequency oscillator 12. The antenna lilis shown radiating energy unidirectionally towards Va modulatedreceiving antenna represented by the block 13. The modulated receivingantenna is coupled to a receiver 14 which may be a narrow bandwidthreceiver having low noise and good stability such as the receivercommercially available from the Collins Radio Company, .identified bythe Model No. 5114. Thesignal detected bythe receiver 14 is then appliedto a signal processing arrangement l5 which, in turn, delivers thesignal to the display element 16. The Fraunhofer pattern is deriveddirectly from the display element 16.

The modulated receiving antenna 13 in this partcula embodiment will beconsidered to comprise a plurality` of v.receiving antennas identifiedby the reference characters 13e, 13b.. 135', and 13Z (FIG. 6) and spacedapartatpreselected intervals to obtain the cosine trigonometricvariation of the range as shown in FIG. 3.

Each antenna is provided .with :an individualamplitude p control meanscomprising` anfattenuator 20 and a phase control means andv theswitching "c'ircuit 22 are typically shownfor the receiving'antennaj13"l and 132, the two antennas that are-atopposite ends oftheantennaV array. The receiving antenna 13 is a corporate fed seriesyofelements arranged toobtain the desiredv trigonometric'relay tionshipalong a line fextending from lthe center of rotationlof the transmittingantennal :-1`Ihe distance from number of elements in the array dependson the accuracy desired in the approximation to the cosinusoidal rangemodulation. reasonable frequency stability and filter bandwidths.Short,'broad-Wall, shunt slot linear arraysof the order of five wavelengths in length oriented transversely to they receiving antenna lengthwith the broad walls of the short arrays ina horizontal plane willprovide the necessary signal level for therharrnonic associated with thefar field pattern. The spacing of the closest short arrays, is,therefore, many wave lengths and the mutual coupling of the elementswill be very small. A dipole antenna may also be utilized, however, itproduces a signal level particularly for the far field patternV that islow and it is desirable to use probes that provide more gain.

The receiver 14 may include a radio frequency filter 23 coupled toreceive the energy from the modulated receiving antenna 13. The radiofrequency -iilter 23 is, in

turn, Vcoupled to a low noise, radio frequency preamplifier 24 whichydelivers' an amplified versionof the signal to a frequency converter ormixer 25. J The mixer 25 is of conventional construction and operates onthe amplifiedsignal; from the radio frequency preamplifier 24, las wellas the oscillator signal delivered by the local oscillator 26 `toproduce a signal of intermediate frequency which is applied to theintermediate frequency stages27. The intermediate frequency from thestages 27 arethen coupled to the signal processor 15. I Itis thoughtthat the radio frequencyiilter 23 is re quired to reduce the amplitudeof the fundamentalRF. Y .frequencyso that the distortion of the harmonicat the frequency w-i-NQ does not occurV during the preampliication andmixing operations. VIt is also thought that this can bel accomplished byusing a preamplifier for the element 24 which limits the gainof thefundamental RF. frequency. Since the signals associated with the farfield,

As few as six elements are required for of 30 kilocycles, and havemodulated gain and good sta bility. e

monic exhibiting the far field dependence from the intermediatefrequency spectrum and to detect and display this information. To thisend, the signal processor comprises a filter 28 coupled to theintermediate frequency stages 27 and delivering its output signal vto adetectorl).

The output signal from lthe detector-v30 is then coupled to aconventional display element 31 which may be a pat.

tern recorder.

ln accordance with the above assumed physical arrange,

ment, the filter 28 may be a crystal filter with a center frequency of30 megacycles-l-30 kilocycles and a bandwidth narrow enough to rejectthe adjacent harmonics. To this end, a 6) decibel bandwidth oflOkilocycles may be used. The detector 30m`ay be a square law detector.-The display element 31 may be in the'form of a com-V mercially availablepattern recorder such as Vthat sold by the Scientific AtlantaCorporation.

The means for controlling the switches 22 comprises a' switch triggeroscillator 32 which may produce squarev Wave pulses at 10 kilocycles andis coupled to a switch programmer 33 having a plurality of outputcircuits'each coupled to an individual switch driver 34 and whichswitchdrivers are individually associated with one of the switching elements22 for the .probes in the modulated receiving i antenna array 13. Theswitch programmer 33 accepts the triggering pulses from theswitchtrigger oscillator 32 and distributes them in a sequential fashion toeach of the switch drivers 34 whereby the switches Vare each vturned onfor thel same lincrement.` The switch driver ,34 mayv.convenientiy'comprise the Beam X Magnetron switching tube soldby theBurroughs Corporation of Detroit, Michif gan. lt .will be recognizedthat the beam switching tubes are responsive ,toV a signal to switchbetween each of its plurality of stable states to provide a uniquesignal in accordance'with theV state to which the beam is sittingandterm of the received radiation is at a very low level, the

low noise preamplifier 24 is thought to be necessary. It should have abandwidth of 30 kilocycles, a noise figure lower than three decibels,and a gain of 30 decibels as well as being very gain stable.

As is well known, the frequency converter or mixer 25 is to translatethe modulation spectrum to an intermediate' frequency spectrum to allowthe far iield term to be more bility of the local oscillator 26 must beof approximately 2 parts in LOQQOO. Therefore, if an intermediatefrequency of 30megacycles is assumedfor the receiver 1 4,

the local oscillator frequency'must be 2,430 megacycles and must,therefore, have a stabilityof 2.5 kilocycles. It is possible to relaxthestability requirements for the local oscillator 26 considerablybykusinga portion of the transmitter signal in combination with averystablelocal y oscillator signal to obtain the localoscillatorfrequency.V V

2l/hen the local oscillatorsignal is vmixed vwith the radiofrequency'signal by means of a local V`mixer to produce` theintermediate frequency signal, the transmitter ,insta-v bility isessentially cancelled out and the stability ofthe,

intermediate frequency is thatofth'e local oscillator.- Y Afurtherrequirementon the mixer 25 isthat it' must-be.

linear over aV decibel dynamic Vrange'at theipowerllevel of `thefarfield 'harmonic'. The intermediate:frequency` stages 27 can be ofconventional-designandi-should preferably be ofrelatively/narrowbandwidth, on theforder `jcan be triggered fromlposition'tc')VK positionto followV eitherV 1O-Y a clockwise orcounter-'clockwise rotation.

The cosinusoidal variation is achieved by switching first the antenna13a. on and then the kantenna 13band, in se'- qu-ential fashion, tol theantenna 132, andV then back through lthe'sarn'e array inV the reversedirection, that is, `from the antenna 132 to 135', et cetera, to 13bto,13a to produce the desired periodic range modulation.

YThe characteristic of the transmitter 10 m-ust also l considered ifaccuratepattern measurements are required. To this end, the power outputofthe transmitter 1 2 must be high enough to provide suiiicient signalfor detection over the desired dynamic rangeand the transmitter must' begain andV frequency stable. The gain and frequency stability of thetransmitter`12 should be comparableto that of the local Aoscillator 26.Excellentfrequencyfcot herence. between `the local oscillator 26 and thetransmit ter V12 1may be obtained by using Ythe transmitterA signalfor'the derivation' of'thelocalv oscillator signal.VV

When the transmitting antenna is rotated, it should-be at a slow raterelative toV the high speed switching rate for theV antenna 13.

. `As Vvv'asfindicated hereinabove, in yorder to effect the A separationofthe farv fieldharmonic from the remainderof Vthe terms` in theSommerfeld expansion, it risnecessary -to multiply the intermediatefrequencyvoltage by a voltl g s. Y

age which is synchronous or `time related with the "form selected fortlrer'ange'y modulation, Vraised to the power ,N +1. Vlin -the abovedescribedgstr-uctural Vorganization .i

thevoltage kmultiplication has been obtained by amplitude controllingthe signalde'tected byeachfof the'receiving H The signal processor 15 isutilized to extract the-har probes 13h13?. In accordancewiththe presentinvention,fthis amplitude control,V Vthat is, theseparate.calibra-y tionofthe amplitude and phase o-f-eachV of the ,signals detectedlbyV the'receiving'.array', Vis separatelycalibratedf 1-with lrespect. totheother receivingprobes'in the rr'ayt);`

produce thedesired.iamplitude-`characteristic@forithr` j j lstages 27and the filter 28 to*` combine these signals and then apply them to fthevfilter 28. This, too, would give each signal portion vin the receivedradiation a unique V amplitude characteristic to allow it to beamplitude filtered.

l. When amplitude control arrangement of FIG.6 is

employed, the voltage multiplication is simulated by calibrating theattenuator 2i) and phase shifter 21 to provide the received signal withdesired amplitude characteristic. In addition,.the calibration procedureprovides a means forcorrecting the effect of the nonuniformity of thereceiving elements of the associated feed and control circuitry. AIt isvery importantgthat the calibration technique for adjusting theattenuator 2t) and phase shifter 21 be maderas simple as possible.Briefly, the amplitude control may be achieved by means of the elements20 and 21 upon calibration to provide in-phase signals with the propervariation in amplitude to produce the desired Vmodulation pattern.

The organization shown in FIG. 7 shows an arrangeiment for vCalibratingthe elementsthrough the use of independently adjustable amplitude` andphase bridges41 and 42 respectively. The amplitude bridge 41 maycomprisea conventional Weinschel Dual Insertion Loss Test .Setup whileraconventional KF. phase bridge'allows adjustment ofthe phase to withinplus or minus one electrical degree.' q

A-s is shown in FIG. 7, the signal received from each element comprisingthe modulated receiving antenna array 13 is compared in turn with asignal from a fixed calibration antenna'identified as the referenceantenna circuit relationshipto the lamplitude bridge, 41 an d the radiofrequency phase bridge 42 at one end terminalthereof." The receivingarray 13 is further. arranged to be v``switched by means of aCalibrating switch 43 from the receiver 14 proper to the Calibratingarrangement and to couple the received signal to the remaining terminalo-f the .amplitude bridge 41 and thephase bridge42. The

' two input' terminals of the amplitude bridge 41 are cach -signals tobe independently adjusted, that is, witho-ut effecting the Yother andthere'by'eliminates the playing back andnforth between the twoxbridges.

vrIhe signals are also coupled tothe phase bridge 42 by means of adirectional coupler 46 connected-to the Calibrating switch 43and-an'attenuator 47 coupledto the reference antenna 40 and, in turn, bymeans of Va magic Tee 4S, to thephase bridge 42. VSince thereceived-radiationmay. not{.be changed,the amplitude of the signalprovided by the reference -antenna Sil is vmodified by* means oftheattenuator 4'I.to adjust forany differences in amplitude between the twosignals prior to their application to the phase bridge 42.. fDuring thecalibration, 'the signal transmitted to the receiving array 13 must beamplitude modulated at 1,000 cycles per second; VA1- ternativ'ely,1a.small antenna may be used asrthe transy .l miting antennaforjcalibration purposes to assure that the receiving array 13" and thereference antenna 40 will,

130 -be inthe far field of this small antenna and then the detectedfield will display the proper dependence with range.V

The calibration procedure to follow includes setting the correct levelof the signal from one of the particular probesl 13a-132, ascalculatedby using the amplitude bridge41. Then the phase of this same receivingprobe is-set by first adjusting the attenuator 47 for the phase bridge42 and then adjusting the phase shifter 21 for the v receiving probetofkset the correct phase. The remaining probes are t-hen adjusted -inthe same fashion and,

upon completion of thekcalibration, the calibration switch 43 isswitched back to the receiver 14 and the actual j measurements can betaken..

With the above Vstructural organization in mind, it should be evidentthat the Fraunhofer pattern may be directlydisplayed :on Ithe element 31`in the form of a strip chart or the like by turning on the switch`trigger 32V whereby the receiving probes 13211-13Z are sequentiallycoupled to -thereceiver 14 to produce the time modulation and lthevoltage multiplication or amplitude control due to the previouscalibration of the attenuators 20 and phase' shifter 21 to provide thethus modulated andrnodied signal to the receiver 14. The receiver 14processes this` signal and produces an intermediate frequencysignallhaving the desired characteristic that is c-oupled to the filter2S. The far field term is then separated out or amplitude filteredby thefilter 28 and the far-field coefiicient of the far field voltage isdetected by the square law detecT tor .Stl: and applied to-.the displayelement .orl pattern recorder 31 to display theamplitude of the far zonef harmonic t-o provide the desired far field pattern.

KIt should-also be noted that it is possible to have the transmittingantenna employed `at lthe receivingend of 49. The reference antenna d@is coupled inl a parallel the circuit, that is, a signal will betransmitted from a plurality of 1antennas having their positionsperiodically f changed in accordancewith the cosine function and thecorresponding location of the transmitting antenna be arranged with alsingle receiving antenna. This arrangement has the advantage that therecording equipment is all wat the main site. In addi-tion, Vthe use ofOW., crontinuous wave radiation, lis preferred since when a pulse systemis employed it includes the further requirement that Ithe transmittingsignal by synchronized with the switching of the probes. i A n Inanother embodiment of the invention the time modulation characteristicmay ybe advantageously employed in combination-with Dopplertechniques-to produce the Fraunhofer pattern. arrangement, however, theDoppler techniques do not produce ya real time computation `in aftashionto allow the Fraunhofer pattern tobe directly obtained from the'apparatus but requires that. the pattern thatv is obtained be passedthrough a Fourier integral computer to obtainithe farfieldpattern. l ,ln

For large antennasmeasurement of the Fraunhofer y pattern vwithin theFresnel regionmay be accomplishedv bysubdividing the antennainto :anumber of elements'such that the measurement is7 in the vfar'zoncof eachelement.l

Therefore," the antenna aperture canbe" divided into N parts, eachlof'whichzis measured separately 'atadistance Rsuchthat Ytherefore,`if'N=10,R0..1IR, resulting in*k asignificant Y anglecharacteristics of the effective-Doppler shifts which are produced.yT-his effect-ivejkDopplershift maybe pro' duced by a .travellingprobewhichy moves in a direction As distinguished,v yfrom the aboveV liltransverse tothe antenna axis or longitudinal width or A a vibratingprobe. Y

in the transverse Doppler arrangement used with very large antennas themodulated receiving antenna array 13 will be noted that the zero Dopplershift will be produced lat a point that is transverse to theantenn-aaperture and.

that Xat all other pointsa 'Doppler shift Vwill be detected.

It is therefore necessary in order to detect the zero Doppler frequencythat a frequency analyzer or lter be employed ltuned to this frequency.A system offthis type is shown in FIG. 8. :antenna aperture in thisfashion, the entireantennais converted from .a spatial function toviatirne function as the probe traverses it at `a constant velocity. Thefrequency lanalyzing-is then accomplished bythe filter and the output ofthe filter is applied to a storage device. At the coml'pletirmof asingle pass across the complete antenna aper-v ture the informationstored in the storageV device is readv out .and applied to a displaydev-ice. Howevenin this instance the pattern displayed is not theFraunhoferpattern of theV aperture and, therefore, it requires furtherprocessing by means of a Fourier 'integral computer `to transform thispattern into the Fraunhofer pattern of the Y aperture. A Y

TheV measuring technique discussed hereinabove maybe employed with `alongitudinally moving receiving probev kto produce `similar Dopplercharacteristics. `The struc- With the sequential' sampling of the.

causey the received energy. to have a periodic time moduandelectronically processing the thusmodulated signal to derive theFraunhofer pattern of the antenna. v

2. A method for measuring the Fraunhoferrpattern of an antenna in otherthan the Fraunhofer' zone comprising Vradiating electromagnetic'en'ergyfrom a source, receiving the radiated energy at a preselected rangeoutside of the YFraunhoferzone in a'periodic time modulated fashion andmodifying thetime modulated patterns to cause the radiated'energy toassume a plur-aiityfof unique information portions including onecharacteristic o'fl'the'Fraunhofer pattern, `and separating ont thesignal portion characteristic of the Fraunhofer pattern. i

V3.'A method for measuring the Fraunhoferpattern of `an antennawithinthe Fresnel zone comprising radiatingVV electromagnetic energy from asourcepin a preselected fashion, positioning at least a single-receiving antenna at a preselected rangefrom 'the source within the`Fresnel zone of 'said source,controlling the receiving antenna to latedcharacteristic,and processing the received energy for separating,out-and displaying the portion thereof repre- ,within the Fresnei zoneof ysaid source, frequency analyz-V ing'the received energy fordetecting Vthe frequency com-k ponent representative of the Fraunhofercharacteristic of tural organization of the receiving circuitry is,however,` l

` modified dueto Ithe different geometric orientation of the antennaandthe receiving probe. With the use of the longitudinal motion of theprobe the Doppler spread corresponding to the aperture extent visconsiderably smaller than in the case of the transverse Doppler andtherefore sentative of the Fraunhofer pattern.

e 4. A'method for measuring the Fraunhofer pattern of an antenna withinthe VFresnel zone comprising radiating electromagnetic energyfromasource in a preselected fashion, positioning at least a singleperiodic time modulatedre-y ceiving antenna at a preselected range from.the source the source, and displaying theFr-aunnofer pattern.

5. A method for determining the fariield characteristic 'i ofaltransmitting antenna ata 'distance which is a fraction of the-actual.far field distance thereof includingrthe steps of radiating a Wsignalfrom'V a transmitting antenna tobe measured, receiving thetransmittedVsignal ata plurality` VVof separate locations in apreselectedtimesequence, the 1 received signal vof theantenna being represented by anexpansion containingray plurality of terms including a term Y`representative of ...the far` eld characteristic, modifying thereceived signal at each receiving location tocause the requires muchnarrower filter .bandwidths to sample the Y aperture extent effectively.

' The vibrating probe type of Doppler arrangement -is characterized bythefact that it does not travel the entire length of the yantennaaperture. -In this instance a coneld.` v

stant Doppler frequency .cannot be detectedl due to the the `.aperturesize. vWith the: vibrating probe type of ar- I rangement, thezero'vDoppler frequency is not detected and the probe -merely traversesthe'points spaced on op- `"posite sides of .the zero Doppler area, orbroad side, to

produce the Doppler shifts for the .various probe positions andY througha frequencyV selection or Vmultiple filtering arrangement :the various-frequenciestjareg detected in ac,l cordance with the particularlocation of the probe. The

outputrsignals from the multiple ltersrare vthen integrated by thesampling integratorto obtain'a pattern similar to large difference inlength between the probe travel land computer to obtain the desiredFraunhofer pattern. It f should beJ noted that in all of thesearrangements theytrarns-. mitting antenna can either be rotated and thepatternf'chari acte'ristics'v determined; orit can remain' stationary.Inl

addition, the 'vibrating probe Atechnique can be vibrated Y,

when the probe is either traveling .transverse or longitudinal Vto Vtheradiation'and processed Vbythe multiple filters..V

. Whatis claimedisz. y l x 1A.I ,A methodformeasuring the Fraunhoferpattern of an antenna in ,otherA than the'Fraunlfiofer zonecomprisin'gradiating electromagnetic euergy'from aource, receivingthejradiatedenergy at a'gpreselected range Voutsideof the Fraunhoferzone in va periodic time. modulated vfashion,

ance with-the sinefonco'sine function.V

portion ofthe signal representative of the far'iield char- Y acteristicto be readily separable, generating an intermedi ate frequencysignal-from thefthusmo'ditied received signal jwhileretaining thesignalcharacteristic of the far field, and t t detecting only the signalportioncharacteristic of the farl 6.. A method for determining the farfield characteristic of a transmitting antenna at a distancewhich is afraction of the actualffareld distance thereof including the steps ofradiating electromagnetic energy'from a transmitting `antenna to bemeasured,po`sitioning 'a plurality of receivi' ingjantennas at apreselected range from the transmitting -1 antenna and spacedapart inaccordance with ka trigonometric function,Y separately couplingtheenergy from each of l the receiving antennas ina preselected timerelationshipto providea `periodic Vtime, modulation of the range,modifying they received energy at each receiving locationY to cause theportioniof kthe venergy y'representative of the lfar field]characteristicto be readily separable, generating an inter-; Y

mediate frequency signal fromthe thus modified received Menergyiwhileretaining the energ'ycharacteristic ofthe far t Y eld,.and il tering.out the portion characteristic` of the far l VA7.` A methodvforvdetermining the fareld characteristic e l l of;V atransmittingantenna Vat a distance whichis a fraction A ofthe actual farfield distance thereof as defined'infclairn 6 wherein thereceiving-antennas are arranged in accoord- 8. A'method for-determiningthe f-ar field VcharacteristicA i v of anantenna'at a distance which isa fraction ofthe actual `j t far V,field dist-ance lincluding thevstepsof transmitting a signal from` the vantenna to be measured,receiving the 'signal from the antennarat a distancel which is 'outsidethe far zone Vof the antenna, periodically varying the distanceV f lanni frnpg;

of the receiving antenna at preselected intervals, multiplying thereceived signals by a voltage which is synchronously related to the timevariations of the distances of the receiving -antenna to allow thereceived radiation to assume a plurality of information channelsincluding a far field characteristic channel to be separable from theother information channels, and filtering the resulting signal to obtainthe sign-al which is representative or" the far field characteristic.

A9. A method for measuring the Fraunhofer pattern of an antenna in otherthan the Fraunhofer zone comprising radiating electromagnetic energyfrom a source, receiving the radiated energy at a plurality of locationsoutside the Fraunhofer zone of the antenna proper and within the Dopplerpattern range, periodic-ally time modulating the received radiatedenergy at each of the loc-ations, and filtering out the desiredfrequency components for determination of the radiation pattern.

10. A method as defined in claim 9 including the steps of processing theresulting radiated energy pattern with av Fourier integral computer toprovide the Fraunhofer pattern.

l1. A method as deiined in claim 9 wherein the radiated energy isreceived ina plane transverse tothe radiation.

12. A method as dened in claim 9 wherein the radiated energy is receivedin a plane extending longitudinally with the radiation.

13. A method as defined in claim 11 wherein the radiated energy isreceived at a plurality of locations extending over a distance less thantheentire length of the antenna aperture and wherein the receiver probeis vibrated over a preselected distance at each location, each receivinglocation being outside the zero Doppler range.

14. Apparatus for determining the far zone characteristic of an antennaby measurements outside the far zone comprising a plurality of alignedreceiving antennas arranged'over a range interval and spaced apartpreselected distances from a transmitting antenna to be measured, eachof said receiving antennas including separate controllable switchingmeans coupled thereto for transmitting a received signal therethroughwhen placed in a signal 1 transmitting mode and normally arranged in anon-transmitting mode, circuit means coupled to each of said switchingmeans for separately and sequentially placing each of said switchingmeans in a signal transmitting mode for a preselected interval toproduce a periodic time modulation of the received radiation, receivingmeans coupled to each of said receiving antennae to receive the signaltransmitted therethrough, said receiving means including means forproducing an intermediate frequency signal, means for providing a signalsynchronously related to thev periodic time modulation of the radiation,means for mixing the intermediate frequency signal and thelattermentioned signal to cause thesignal portion representative of the fareld characteristic to be readily separable, and filtering meansconnected to be responsive to-the signalr from the mixing meansincluding the far iield characteristic.

15. Apparatus of-the type of claim 14 wherein the receivingantennas arespaced apart in accordance with a preselected trigonometric functionover the range and the transmitting intervals of each of the antennas'are controlled tobe substantially equal to provide the periodic timemodulation of the received radiation.

16. Apparatus of the type of claim 14 wherein the receiving antennas arespaced apart substantially equal distances and the transmittingintervals of each of the anx tennas are dened to occur in unequalincrements of time e to provide the periodic time modulation of thereceived lfar zionecomprising a plurality of aligned receiving an-Vtermas arranged over a range interval and spaced apart preselecteddistances from a transmitting antenna to be measured, each of saidreceiving antennas including separate controllable switching meanscoupled thereto for transmitting a received signal therethrough whenplaced in a signal transmitting mode and normally arranged in anon-transmitting mode, circuit means coupled to each of said switchingmeans for separately and sequentially placing each of said switchingmeans in a signal transmitting mode for a preselected interval toproduce a periodic time modulation ofthe received radiation, each ofsaid receiving antennas further including a series connected attenuatorand phase shifter calibrated to cause the time modulated radiation to bemodified whereby the signal portion characteristic of the far zone maybe separated, receiving means coupledto each of saidy receiving antennato receive the signal transmitted therethrough, said receiving meansincluding means for producing an intermediate frequency signal, andfiltering means connected to be responsive to the signal portion fromthe mixing means including the far iield characteristic.

i8. Apparatus as defined in claim 17including a detector coupled to saidiilter to detect and separate out the far field characteristic only, anddisplay means coupled to said detector for recording and display-ing thefar zone pattern. y

19. Apparatus for determining the far zone characteristic of atransmitting antenna within the Fresnel region comprising a plurality ofreceiving antennas spaced apart in accordance with a preselectedtrigonometric distribution over the range interval, individual switchingmeans coupled to each receiving antenna to selectively control therespective antenna, switch programming means coupled to each of saidswitching means to control the switching time increments of each antennawith reference to the spacing between the adjacent antennas to produce aperiodic time modulation of received radiation, each of said antennasincluding a series arrangement of a calibrated attenuator and phaseshifter coupled intermediate the antenna and the switching means tocause the signal components of the received radiation to each assume aunique signal characteristic and including a signal por-tion having thevfar zone characteristic, a receiver including a frequency mixing meanscoupled by a corporate feed with each of said switching means to receivesignals from the receiving antennas during the switching time incrementsf of each antenna, a local oscillator coupled to said mixing means incombination with the received signal to produce an intermediatefrequency signal, filtering means dened to be responsive to the signalcomponent representative of the far zone characteristic,y and meanscoupled to said filtering means for recording the signal characteristicof the far Zone.

for coupling one of the signals to be measured to one terminal of eachof said bridges, means for coupling the other signal to be measured toythe other terminals of each of said bridges, an amplitude adjustingmeans coupled in series circuit arrangement with one of the terminalsfor the phase bridge to allow the amplitude of the corresponding signalto be adjusted.

References Cited in the le of ythis patent Bickmore: Fraunhofer PatternMeasurement inthe Fresnel Region, published in the Canadian Journal ofPhysics, v01. 35, 1957, pp. 1299-1308.

1. A METHOD FOR MEASURING THE FRAUNHOFER PATTERN OF AN ANTENNA IN OTHER THAN THE FRAUNHOFER ZONE COMPRISING RADIATING ELECTROMAGNETIC ENERGY FROM A SOURCE, RECEIVING THE RADIATED ENERGY AT A PRESELECTED RANGE OUTSIDE OF THE FRAUNHOFER ZONE IN A PERIODIC TIME MODULATED FASHION, AND ELECTRONICALLY PROCESSING THE THUS MODULATED SIGNAL TO DERIVE THE FRAUNHOFER PATTERN OF THE ANTENNA. 